A semiconductor dynamic quantity sensor has been disclosed in JP-A-11-326365. FIGS. 5A–5B schematically show the structure of the semiconductor dynamic quantity (acceleration) sensor disclosed in the Patent Document 1. Specifically, FIG. 5A is a plan view of this sensor element, and FIG. 5B is a cross-sectional view taken along a line VB—VB of FIG. 5A.
As shown in FIGS. 5A and 5B, the sensor comprises a semiconductor substrate (silicon substrate) 1, an insulating layer 2 and a semiconductor layer (silicon layer) 3 formed on the insulating layer 2.
The semiconductor layer 3 is patterned in a pattern style as shown in FIG. 5A by a well-known photolithography technique or the like, and the insulating layer 2 is selectively etched and removed in an area Z2 indicated by a broken line of FIG. 5A to form a groove portion. That is, the semiconductor layer 3 is formed so as to be floated from the substrate 1 in the area Z2 corresponding to the groove portion and supported through the insulating layer 2 by the substrate 1 in the other area.
Specifically, a mass (weight portion) 320 derived from the semiconductor layer 3, comb-shaped movable electrodes ME11 and ME12 formed integrally with the mass 320, fixed electrodes FE11 and FE12, each of which is supported at one end thereof so as to confront each movable electrode, and beams B11 to B14 for supporting the mass 320 at one ends thereof are formed within the area Z2. Furthermore, anchor portions 321, 322, 331 and 332 derived from the semiconductor layer 3 are formed out of the area Z2.
The other ends of the respective beams B11 to B14 are supported by the anchor portions 321 and 322, and the fixed electrodes FE11 and FE12 are supported by the anchor portions 331 and 332, respectively. An electrode pad 410 formed of metal such as aluminum or the like is formed on the anchor portion 321 for outputting the potential of the movable electrodes ME11 and ME12. Furthermore, electrode pads 411 and 412 formed of metal such as aluminum or the like are formed on the anchor portions 331 and 332 to supply voltages to the fixed electrodes FE11 and FE12, respectively.
Here, the mass 320 is designed so that the beams B11 to B14 allow displacement of the mass 320 in the Y-axis direction, but restrains displacement in the X-axis direction of the mass 320 as shown in FIG. 5A.
Accordingly, when acceleration is applied to the mass 320 in the Y-axis direction, the movable electrodes ME11 and ME12 integrally formed with the mass 320 together with displacement of the mass 320 in the Y-axis direction are likewise displaced in the Y-axis direction. In this case, the distance between the movable electrode ME11 and the fixed electrode FE11 is increased or reduced while the distance between the movable electrode ME12 and the fixed electrode FE12 is reduced or increased. That is, with this sensor, the variation of the distance between the electrodes as described above is detected as variation of the electrostatic capacitance CS1 or CS2, and the variations of the electrostatic capacitance CS1, CS2 are output as a voltage value through, for example, a switched capacitor circuit shown in FIG. 6. The direction and magnitude of the acceleration applied are detected on the basis of the voltage value thus taken out.
The construction and operation of the switched capacitor circuit will be briefly described with reference to FIGS. 6 and 7.
In FIG. 6, reference numeral 100 represents an equivalent circuit of the semiconductor quantity sensor. Reference numeral 200 represents a switched capacitor circuit. Here, a terminal P0 of the circuit 100 corresponds to the electrode pad 410 of the semiconductor quantity sensor, and terminals P1 and P2 correspond to the electrode pads 411 and 412 of the semiconductor quantity sensor, respectively. The potential (charge) output through the terminal P0 (electrode pad 410) is input to the switched capacitor circuit 200.
The switched capacitor circuit 200 comprises an operational amplifier OP, and a capacitor Cf and a switch SW which are connected to each other in parallel in the feedback path of the operational amplifier OP. A signal output from the terminal P0 is input to the inverting input terminal of the operational amplifier OP, and a half voltage of a voltage Vcc applied between the terminals P1 and P2 of the circuit 100 (sensor), that is, a voltage “Vcc/2” is applied to the non-inverting input terminal of the operational amplifier OP.
Next, the operation of the circuits 100 and 200 will be described in combination with the timing chart of FIG. 7.
As shown in FIG. 7, alternating signals (voltages) each of which alternates between a voltage “0” V and a voltage Vcc, for example, at a frequency of 50 kHz to 150 kHz are stationarily applied to the terminals P1 (electrode pad 411) and P2 (electrode pad 412) of the circuit 100 (sensor) respectively while the alternating signals are opposite in phase to each other. Furthermore, the ON/OFF operation of the switch SW of the switched capacitor circuit 200 is controlled in synchronism with the alternating frequency of each alternating signal (voltage).
Accordingly, for example during the period between the timings T1 and T2, the switch SW is turned on, so that the output voltage Vo of the switched capacitor circuit 200 is maintained at the voltage of “Vcc/2”. During the period between the timings T2 and T3, the switch SW is turned off. Therefore, the difference between the variations of the electrostatic capacitance CS1 and CS2 in connection with the application of the acceleration described above, accurately, the difference between the variations in connection with the inversion of the voltage applied between the terminals P1 and P2 is charged in the capacitor of the switched capacitor circuit 200 through the terminal P0. As a result, the output voltage Vo from the switched capacitor circuit 200 is represented as follows:Vo=(CS1−CS2)·Vcc/Cf
Accordingly, the output voltage Vo has the potential corresponding to the capacitance difference (CS1−CS2) of the electrostatic capacitance CS1 and the electrostatic capacitance CS2 as indicated.
The output voltage Vo is properly sampled while the operation as described above is repeated, and only low-frequency components of, for example, 500 Hz or less are extracted from the sampling values, thereby achieving the value corresponding to the acceleration applied to the sensor.
In addition to the sensor for detecting application of a dynamic quantity (acceleration) in one axis direction (uniaxial direction) as described above, a sensor for detecting application of a dynamic quantity (acceleration) in two orthogonal axis directions (biaxial directions) as disclosed in U.S. Pat. No. 5,880,369 (See FIG. 4 of this Patent) has been disclosed. FIGS. 8A and 8B schematically show the structure of a semiconductor dynamic quantity (acceleration) disclosed in U.S. Pat. No. 5,880,369 (FIG. 4). FIG. 8A is a plan view showing this sensor element, and FIG. 8B is a cross-sectional view taken along a line VIIIB—VIIIB.
As shown in FIGS. 8A and 8B, this sensor also comprises a semiconductor substrate (silicon substrate) 1, an insulating layer 2 and a semiconductor layer (silicon layer) 3 formed on the insulating layer 2.
The semiconductor layer 3 is patterned in a pattern style as shown in FIG. 8A by a well-known photolithography technique or the like, and the insulating layer 2 is selectively etched and removed in an area Z3 indicated by a broken line of FIG. 8A to form a groove portion. That is, the semiconductor layer 3 is formed so as to be floated from the substrate 1 in the area Z3 corresponding to the groove portion and supported through the insulating layer 2 by the substrate 1 in the other area.
Specifically, a mass (weight portion) 340 derived from the semiconductor layer 3, comb-shaped movable electrodes ME21 to ME24 formed integrally with the mass 340, fixed electrodes FE21 to FE24, each of which is supported at one end thereof so as to confront each movable electrode, and beams B21 to B28 for supporting the mass 340 at one ends thereof are formed within the area Z3. Furthermore, beam fixing portions S21 to S24 and anchor portions 351 to 354 derived from the semiconductor layer 3 are formed out of the area Z3.
The other ends of the respective beams B21 to B28 are supported by the beam fixing portions S21 to S24, and the fixed electrodes FE21 to FE 24 are supported by the anchor portions 351 to 354, respectively. An electrode pad 420 formed of metal such as aluminum or the like is formed on the beam fixed portion S22 to output the potential of the movable electrodes ME21 to ME24. Furthermore, electrode pads 421 to 424 formed of metal such as aluminum or the like are formed on the anchor portions 351 to 354 to supply voltages to the fixed electrodes FE21 to FE24, respectively.
Here, the mass 340 is designed so that the beams B21 to B24 allow displacement of the mass 340 in the X-axis direction, and the beams B25 to B28 allows displacement of the mass 340 in the Y-axis direction as shown in FIG. 8A.
Accordingly, for example when acceleration in the X-axis direction is applied to the mass 340, the movable electrodes ME21 and ME22 formed integrally with the mass 340 are likewise displaced in the X-axis direction in connection with the displacement of the mass 340 in the X-axis direction. In this case, the distance between the movable electrode ME21 and the fixed electrode FE21 is increased or reduced while the distance between the movable electrode ME22 and the fixed electrode FE22 is reduced or increased. Furthermore, when acceleration in the Y-axis direction is applied to the mass 340, the movable electrodes ME23 and ME24 formed integrally with the mass 340 are likewise displaced in the Y-axis direction in connection with the displacement of the mass 340 in the Y-axis direction. In this case, the distance between the movable electrode ME23 and the fixed electrode FE23 is increased or reduced while the distance between the movable electrode ME24 and the fixed electrode FE24 is reduced or increased.
With this sensor, the variation of the distance between the respective electrodes as described above is detected as the variation of the electrostatic capacitance, and the variations in electrostatic capacitance in the X-axis direction and the Y-axis direction are converted to voltage values through the switched capacitor circuit shown in FIG. 6.
As described above, according to the semiconductor dynamic quantity sensor shown in FIGS. 8A and 8B, it is possible to detect the application of the dynamic quantities (acceleration) in the two orthogonal axis directions. However, the conventional sensor as described above adopts such a structure having a low degree of freedom that one mass is commonly supported by two kinds of beams to allow the displacements of the mass in the two axis directions.
Specifically, as shown in FIG. 8A, when displacement of the mass 340 in the X-axis direction is considered, the beams B21 to B24 are disposed to allow the displacement of the mass 340 in the X-axis direction. On the other hand, the other beams B25 to B28 are disposed to restrain the displacement of the mass 340 in the X-axis direction. Conversely, when the displacement of the mass 340 in the Y-axis direction is considered, the beams B25 to B28 allow the displacement of the mass 340 in the Y-axis direction whereas the beams B21 to B24 restrain the displacement of the mass 340 in the Y-axis direction.
As described above, the conventional semiconductor quantity sensor is designed so that the application of the dynamic quantity (acceleration) in the orthogonal two axis directions can be detected. However, the displacement of the mass in the one axis direction intervenes in the displacement of the mass in the other axis direction. This will be referred to as one axis intervening and reducing other-axis sensitivity. Therefore, in a conventional semiconductor dynamic quantity sensor, a reduction in other-axis sensitivity is unavoidable.